1. Field of the Invention
The present invention relates to an apparatus and method for printing letters and patterns on an insulator of a spark plug.
2. Description of the Related Art
A conventional printing apparatus 9 as shown in FIG. 25 is used for printing letters and patterns on a surface of a spark plug 5. The letterpress printing apparatus 9 has: an ink roller 91 rotating around a not-shown rotating axis; and other rollers.
Concretely, the printing apparatus 9 comprises: an ink tank 90 for preserving an ink 900; an ink roller 91 for transferring a not-shown ink film through a transferring unit 92 to an ink kneading roller 931; and ink kneading rollers 932 to 936 for adjusting a viscosity and thickness of the ink film.
The letterpress printing apparatus 9 further comprises: a letterpress roller 94 with a letterpress 941 for receiving the ink film from the ink kneading roller 936; and a transferring roller 95 with a transferring surface 950 for receiving the ink film formed on the letterpress 941.
The pattern of letterpress 941 is almost the same as a pattern to be formed on the surface of the spark plug 5. Thus, the ink film transferred from the ink kneading roller 936 is formed on the letterpress 941. Thus, an ink film corresponding to the printing pattern is formed on the letterpress 941. Further, the letterpress roller 94 is made of a rubber.
Further the transferring unit 92 comprises: the ink roller 91; a roller 921 which alternately contacts the ink roller 91 and ink kneading roller 931; and an arm 922 for supporting the roller 921 at a rotating roller 923. In other, words, the roller 921 moves along the arm 922 as a radial arm around the rotating roller 923.
The spark plug insulator 5 is a cylindrical ceramic product with a small radius. Therefore, the outer surface becomes a steep slope. Therefore, the conventional letterpress printing apparatus 9 has a disadvantage that the letterpress 941 is worn away at convex portions, due to contacts with the outer surface of the spark plug insulator. Accordingly, the shape of the letterpress 941 is decaying every printing process and the printing quality becomes degraded, due to the change in the ink film pattern.